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Preparation of Solid Superacid Catalysts

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Preparation of Solid Superacid Catalysts 石 油 学 会 誌 Sekiyu Gakkaishi, 39, (3), 185-193 (1996) 185 [Review Paper] Preparation of Solid Superacid Catalysts Kazushi ARATA Dept. of Science, Hokkaido University of Education, Hachiman-cho, Hakodate, Hokkaido 040 (Received September 1, 1995) Our recent works on preparation of solid superacid catalysts, sulfate-metal oxides and tungstate-, molybdate-, or borate-metal oxides, are reviewed. Superacids with an acid strength of up to Ho≦-16.04 are obtained by adsorbing sulfate ion onto amorphous oxides of Fe, Ti, Zr, Hf, Sn, and Si followed by calcination in air at above 500℃; a superacid of Al2O3 is prepared from the crystallized oxide. Superacids by metal oxides with an acid strength of up to Ho≦-14.52 are synthesized in the same manner as those of the sulfate superacids by supporting ZrO2 with WO3 (13wt% W) or MoO3 (6wt% Mo): impregnation of amorphous ZrO2 with aqueous ammonium metatungstate or molybdic acid dissolved in ammonia water, followed by evaporation of water and calcination in air at 800-850℃. Similar superacids are prepared by impregnation of amorphous oxides of Sn, Ti, and Fe with ammonium metatungstate and calcination in air, the maximum activity for reaction of isopentane being observed with calcination at 1000℃ for the Sn base material and 700℃ for those of Ti and Fe; borate supported on zirconia also shows superacidity, though weak in strength. 1. Introduction their catalytic actions, sulfate-supported metal oxides and oxides such as tungsten and molyb- With reference to Gillespie's definition, an acid denum oxides supported on metal oxides; in that is stronger than Ho=-12, which corresponds particular, the article focusses on procedure for to the acid strength of 100% H2SO4, is known as a preparation of solid superacids. superacid1),2). Among a large number of solid acids, SiO2-Al2O3 bears strong acid sites on the 2. Superacids of Sulfated Metal Oxides surface and its highest acid strength has been determined, so far, to be Ho=-12, whose value is 2.1. Preparation of SO4/Fe2O3, TiO2, ZrO2 in the range of superacidity. In the field of cata- 2.1.1. Preparation of Zr(OH)48),9) lytic chemistry, solid acid systems stronger than Two hundred grams of ZrOCl2・8H2O (guaran- acidic oxides such as SiO2-Al2O3 and zeolites have teed grade, Nakarai Chemicals, Ltd.) are dissolved been developed recently, which are place in the into 2.5l of distilled water in a 5l beaker, and category of solid superacids. ammonia water (28%) is added by dropping into In the 1980s, we studied synthesis of solid the aqueous solution, while stirring, until pH8 of superacid catalysts with acid strengths of up to the solution is attained, measured with a slip of pH Ho≦-16.04 on the surfaces of oxides of Fe, Ti, Zr, test paper. The precipitated mixture is allowed to Hf, Sn, Si, and Al by strong coordination of sulfate stand for a day, after further stirring for 30min. ion. The superacids were satisfactorily active in a The aqueous portion is decanted away from the heterogeneous system for reactions which are precipitates, and fresh water is added, followed by generally catalyzed by strong acid, especially by stirring, settling down, and decanting away the superacids, such as SbF5-HF and SbF5-FSO3H. aqueous portion; washing of the precipitates by Following the same procedure as those of the decantation is repeated until the total amount of sulfated superacids, we have also synthesized water used is 60l, after which almost no chloride another type of superacid, not containing any ions are detected in the washing. The precipitates sulfate matter but consisting of metal oxides, are finally isolated by filtration and dried at 100℃ which can be used at high temperatures over for 24h. 500℃. The hydroxide is prepared in the manner We have published several reviews concerning described above, from ZrO(NO3)2・2H2O (Nakarai the superacids3)-7). This review summarizes our Chemicals, Ltd., extra pure reagent) as a starting recent work on syntheses of solid superacids and material. Since residual nitrate ions are thermal- 石 油 学 会 誌 Sekiyu Gakkaishi, Vol. 39, No. 3, 1996 186 ly decomposed away, thorough washing of the finally sealed in an ampoule, while being hot, precipitates with distilled water, as mentioned until use. The superacid catalysts thus prepared above, is not necessary; three times would suffice from ZrOCl2, ZrO(NO3)2, Ti[OCH(CH3)2]4, and for the decantation by washing, thus in this point TiCl4 are referred to as SO4/ZrO2-I, SO4/ZrO2-II, the preparation being made easier. SO4/TiO2-I, and SO4/TiO2-II, respectively; the 2.1.2. Preparation of H4TiO48),9) catalysts from thermal decomposition and hydrol- A 290ml volume of Ti[OCH(CH3)2]4 (Wako ysis of Fe(NO3)3 are referred to as SO4/Fe2O3-I and Pure Chemical Industries, Ltd.) is added into 2l -II, respectively. of distilled water while stirring in a 5l beaker, and The catalysts can be also obtained by treatment the white precipitates formed are dissolved by with ammonium sulfate, but the catalytic activity gradually adding 250ml of conc. HNO3, while is usually lower than that with sulfuric acid. As stirring. Ammonia water (28%), -300ml, is stated above, the catalyst is usually calcined in a added into the aqueous solution while stirring, Pyrex tube and sealed in an ampoule until use to until the solution attains pH8, followed by further avoid humidity. The appearance of the catalysts stirring for 30min, allowed to stand for a day, with the sulfate treatment differs greatly from that followed by washing the precipitates by decan- without the treatment. The former catalysts are tation of the 5l beaker twice, filtering, and finally finely powdered solids which coat the wall of a dryed at 100℃ for 24h. glass ampoule obscuring vision, whereas the latter Another method of preparing H4TiO4 is by is not. Thus, it can be confirmed whether or not hydrolyzing TiCl4 as follows: 80ml of TiCl4 superacidity has been generated. Coating the (Wako Pure Chemicals) are gradually added into wall of the glass ampoule is, however, not the case 2l of distilled water in a 5l beaker cooled by when the sample has adsorbed moisture from the ice water, forming large amounts of HCl gas. air. The catalysts obtained from isopropoxide of Ammonia water is added until pH8 (at room Ti and nitrates of Fe and Zr as starting materials are temperature) is attained, followed by the above high in activity and easy to prepare. procedures; the precipitates are washed thoroughly 2.1.5. Catalytic Activities by decantation using 60l of water until no chloride The SO4/Fe2O3 catalysts were examined in the ions are detected in the filtrate. The aqueous dehydration reaction of ethanol as shown in portion might become cloudy during washing, but Table 110); SO4/Fe2O3-III was prepared by the the white washing can be decanted away. hydrolysis of FeCl3 as a starting material, and 2.1.3. Preparation of Fe(OH)3 and Amorphous followed by the treatment and calcining at 500℃. Fe2O38),9) The SO4/Fe2O3-II and -III catalysts, treated with Five hundred grams of Fe(NO3)3・9H2O (Wako 0.05-0.5M H2SO4 or (NH4)2SO4, showed quite Pure Chemicals, guaranteed reagent) are dissolved high activities, much higher than that of SiO2- in 2l of water in a 5l beaker, followed by hydro- Al2O3, which is well known as one of the catalysts lyzing with ammonia water (-300ml used, pH 8), having highest surface acidity. washing, and drying as explained above. The The catalytic action for the reaction of butane decantation washing is performed until the liquid which is catalyzed by superacids was examined, portion becomes cloudy (7-8 times). and it was found that the present catalysts are active Amorphous Fe2O3 is prepared by thermally for the skeletal isomerization of butane to iso- decomposing Fe(NO3)3・9H2O at about 200℃ or butane, at room temperature or even at 0℃, as higher; brown fuming gas is generated by shown in Fig. 111). SiO2-Al2O3 was totally inac- decomposition of iron nitrate after fusion of the tive for the reaction. Acid strength of SiO2-Al2O3 nitrate, and solid iron oxides are obtained upon used was in the range of -12.70<Ho≦-11.35. heating for 3-6h. Consequently, the SO4/Fe2O3 catalysts are con- 2.1.4. Sulfate Treatment8),9) cluded to be solid superacids. The materials prepared above are powdered to SO4/Fe2O3-III was deactivated below 5% of con- below 100-mesh, followed by pouring 30ml of version, though the yield of isobutane increased aqueous sulfuric acid (0.5M concentration for Ti with time over SO4/Fe2O3-I and -II. The material and Zr base materials and 0.25M for Fe) on to 2g of prepared by hydrolyzing FeCl3 with ammonia the dried hydroxides or oxides on a filter paper, followed by treating with H2SO4 and calcining had and drying in air; the iron materials are again a tendency to be converted into iron sulfate; the powdered because of solidifying after drying. catalyst obtained by using 0.05M showed activity The materials are calcined in a Pyrex glass tube in higher than that using 0.25M for dehydration of air at temperatures above 500℃ (600-650℃ for Zr ethanol, as shown in Table 1, and only the catalyst samples, 525℃ for Ti, 500℃ for Fe) for 3h and prepared by treatment with 0.05M showed activity 石 油 学 会 誌 Sekiyu Gakkaishi, Vol. 39, No. 3, 1996 187 Table 1 Dehydration of Ethanol over SO4/Fe2O3 at 250℃ a) Product and selectivity: 57-67% ethyl ether and 33-43% ethylene.
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